A high basal phosphodiesterase (PDE) activity in sinoatrial node cells (SANC) regulates cAMP and modulates cAMP-mediated PKA-dependent phosphorylation of Ca2+ cycling proteins that determines the characteristics of local subsarcolemmal Ca2+ releases (LCR). LCRs occur during the terminal diastolic depolarization and activate the inward Na+-Ca2+ exchange current, thus, regulating the SANC basal spontaneous beating rate. We used perforated patch to record spontaneous action potentials (APs) in conjunction with confocal linescan images or whole-cell patch clamp to measure L-type Ca2+ current (ICa,L) at 35oC. A specific PDE3 inhibitor, cilostamide (Cil, 0.3 mol/L), increased the spontaneous beating rate by 28% (from 154 21 to 191 17 beat/min); in contrast a specific PDE4 inhibitor, rolipram (Rol, 2 mol/L), had no effect on the firing rate. The substantial role of PDE4 in the control of basal spontaneous SANC firing was unmasked when PDE3 and PDE4 were simultaneously inhibited: the combination of Rol and Cil produced 57% acceleration of the beating rate (from129 10 to 200 10 beat/min) equivalent to that produced by the broad-spectrum PDE inhibitor, IBMX (100gM), i.e. 55% (from 145 8 to 221 6 beat/min). The combination of Cil and Rol increased LCR amplitude, size and decreased the LCR period similar to IBMX. Both IBMX and the combination of Cil and Rol produced a substantial increase in ICa,L 135% (from 10 2 to 22 3 pA/pF) and 125% (from 6 1 to 13 3 pA/pF) respectively. The efficiency of PDE3 inhibition alone on spontaneous beating rate might be ascribed to its lower Km for cAMP, than that of PDE4. When PDE3 is suppressed level of cAMP is sufficiently elevated to activate PDE4. Therefore, in the presence of PDE3 inhibition, PDE4 inhibition contributes to the increase in the spontaneous beating rate. Thus, an interaction of basal PDE3 and PDE4 activities in SANC modulates cAMP level and ICa,L which controls Ca2+ influx, cell Ca2+ load, determining LCR characteristics. When both PDE3 and PDE4 are inhibited the maximum spontaneous beating rate of SANC is achieved.